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Mechanical alloying for high performance aluminium alloys Inco Alloys International Huntingdon, West Virginia, WV 25720, USA Tel: +1 (304) 526-5100 Abstract Powder atornisaUon and mechanical alloying are outlined Introduction In powder atomisation, molten metal is poured through high pressure streams of inert gas, which atomise the metal into fine powder of uniform composition The alloyed powder is consolidated by hot isostatic pressing It can be pressed either into shaped components or into billets for further working Mechanical treatment can create alloys of metals that cannot be obtained by other means The process involves high energy milling of dry powders in a ball mill The powder particles are continually welded together and broken apart until they become homogeneous The alloyed powder is consolidated into billets by hot extrusion or pressing The history of the mechanical alloying process dates back to the early 70s, when the process began to be developed for three main alloy systems, based on nickel, iron and aluminium respectively Mechanical alloying is a high energy, dry milling, powder metallurgy operation In the production of the aluminium alloys, a process control agent is used to form dispersoids; the powder compact for subsequent hot working is formed by vacuum hot pressing The compacted alloy is a very fine grain material stabilised by a fine dispersion of oxide and carbide Because this structure provides added strength, little or no intermetallic precipitation is necessary in developing the alloy properties, which are, therefore, homogeneous through a thick section or forging Powder production Horizontal ball mills and agitators are used in the production of aluminium powders Raw material charges for mechanically alloyed aluminium processing consist of elemental or pre-alloyed powders, master alloys and organic milling aids to control the balance between cold welding and grinding, the central feature of the mechanical alloying process The whole milling operation is carried out in a protective atmosphere Degassing equipment is used to remove volatiles from the organic agents and moisture from the hydrated surface oxides on the powder particles Magnetic cleaning under inert gas shielding is used to remove any particles of iron-base materials that may result from the wear of the mills and the grinding media Cleanliness is critical, for raw materials to finished 38 products It is achieved through special containment and handling equipment and inert gas shielding Consolidation The degassed mechanically alloyed powders are heated under vacuum in cylindrical steel dies Rams, acting through the top and bottom of the vacuum vessels, compact the powder to 99-100% density The compacted products include billets of 280mm diameter weighing 90kg and billets of 470mm diameter weighing 540kg Although essentially fully dense, the vacuum hot pressed billets have to be extruded to complete the consolidation process Because aluminium particles are always coated with an oxide film, which is not greatly dispersed during hot pressing, prior particle boundaries in the billet contain oxide films which impair ductility Plastic deformation during extrusion ruptures these films and converts them to dispersed particles, completing the full integrity of the material Extruded bar stock is typically 150mm diameter, or less, from the 470mm diameter billet and 90mm diameter, or less, from the 280mm billet When material is to be forged, larger bar sizes can be extruded if the amount of subsequent forging deformation is adequate In these circumstances, bar sizes up to 290mm diameter can be used The extrusion operation is carried out on a contract basis by specialist extruders Extrusions of appropriate size can be made directly from the vacuum hot pressed billet Smaller shapes are re-extruded from large extruded bar stock Conversion of the bar stock to other forms is accomplished by conventional means Alloys in production There are two alloys already at production status: lncoMAP alloy AL-9052 is a strong, corrosion-resistant aluminium alloy with 4% magnesium, targeted primarily at the marine industry: lncoMAP alloy AL-950XL, with 4% magnesium and 1.3% lithium in an aluminium base, is a low density, high stiffness product intended f o r aerospace forgings A third material, designated IncoMAP alloy AL 9021 is ready for production With 4% copper and 1.5% magnesium, its main projected application is for the matrix of highly rigid composites containing fine particulate SiC MATERIALS & DESIGN Vol 10 No JANUARY/FEBRUARY 1989